drm/xe/display: Add frontbuffer setter/getter for xe_bo

Xe is not carrying frontbuffer pointer in xe_bo. Define it's getter as
NULL.  Setter simply returns pointer which was provided as a
parameter.

v3: Do not take any references
v2: Handle xe_bo_put as well
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/compat-i915-headers/gem/i915_gem_object_frontbuffer.h

+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright © 2022 Intel Corporation
+ */
+
+#ifndef _I915_GEM_OBJECT_FRONTBUFFER_H_
+#define _I915_GEM_OBJECT_FRONTBUFFER_H_
+
+#define i915_gem_object_get_frontbuffer(obj)		NULL
+#define i915_gem_object_set_frontbuffer(obj, front)	(front)
+
+#endif